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114                         METALLURGY OF IRON AND STEEL.
the silicon. This is true in both the acid and the basic processes, but the elimination is less certain in the acid process, for part of the silicon is sometimes left after the carbon is burned, if there has been an excessive temperature at an early stage of the operation. In the basic converter the incomplete combustion of silicon does not occur, owing to three reasons:
(1)   The silicon is lower in the pig, because the oxidation of phosphorus is relied upon for heat.
(2)  Burned lime is added before blowing, to seize the silica as soon as formed and prevent cutting of the lining, and the heating and melting of this lime absorbs so much heat that the critical temperature cannot well be reached.
(3)   The basic slag has a greater affinity for silica than the sili-cious slag of an acid converter, and it is probable that under these conditions the critical temperature is raised.
When the silicon is eliminated, the carbon begins to burn and continues until there is only about .05 per cent., while the manganese follows the same course that it does in acid work, part of it being eliminated while the silicon is burning and another part during the combustion of carbon. The proportion of manganese at any particular time will depend upon the original percentage in the pig,, but, comparing similar contents, the amount eliminated •will be less than in the acid practice, for there is less demand for its oxide in a basic slag, and the inducements to oxidation are, therefore, taken away.
SEO. Vllb.—Elimination'of phosphorus.—With the exception of the basic lining, which is supposed to remain inert, and the basic slag, which has no chance in the early part of the operation to do anything besides aid slightly in "the burning of silicon and retard slightly the oxidation of manganese, the reactions in the metal in a basic converter are almost identical with the reactions in the acid vessel up to the point when the carbon is reduced-to .05 per cent. From this point comparison ceases, for there the acid process ends, while the basic begins the characteristic chapter in its history in the elimination of phosphorus and sulphur.
In an acid heat phosphorus is present to a certain extent, and, if blowing were continued, it may be supposed that at the very surface of an air-bubble phosphoric acid would be formed which, rising through the metal, would unite with oxide of iron and form